The US has said it will wait for details of a Russian proposal on Syrian chemical weapons but warned it would not permit "delays and avoidance".
US Secretary of State John Kerry said the plan must be "swift and verifiable" and warned its implementation would be "exceedingly difficult".
Russia's plan would see the chemical weapons put under international control, with the aim of destruction.
Syria has said it accepts the Russian proposal, though details are sketchy.
France earlier said it would put a strong resolution to the UN Security Council on the control and subsequent destruction of the chemical weapons.
Foreign Minister Laurent Fabius said the resolution would threaten "extremely serious" consequences if Syria breached its conditions.
The US claims that Syrian government forces carried out a chemical weapons attack in Damascus on 21 August, killing 1,429 people.
Mr Assad's government blames the attack on rebels fighting to overthrow him, in a conflict that the UN says has claimed some 100,000 lives.
'Hard look'Mr Kerry said that if the UN were used as the vehicle for pursuing the Russian plan, it must not become a debating society.He told a hearing of the House of Representatives Armed Services Committee the US was waiting for details of the Russian proposal, "but we're not waiting for long".
He said: "President Obama will take a hard look at it. But it has to be swift, it has to be real, it has to be verifiable.
"We have to show Syria, Russia and the world we are not going to fall for stalling tactics."
Mr Kerry urged Congress to stand by Mr Obama, saying the president was not asking for a declaration of war, simply for the power to show that the US "means what we say".
There have been few details so far of Russia's plan, but Foreign Minister Sergei Lavrov said earlier in Moscow that it was "preparing a concrete proposal which will be presented to all interested sides, including the US... a workable, specific, concrete plan".
Syrian Foreign Minister Walid Muallem, who is in Moscow, was quoted by Russian news agency Interfax as saying: "We held a very fruitful round of talks with [Mr Lavrov] yesterday and he proposed an initiative relating to chemical weapons. And in the evening, we agreed to the Russian initiative."
This would "remove the grounds for American aggression", he said.
The US Senate had been expected to vote this week on a resolution authorising military force, but the Russian plan has led to a postponement.
src BBC
